This invention relates to systems for transmitting information from the bottom of a bore hole to the surface by way of pressure pulses created in a circulating mud stream in the drill string. More particularly, this invention relates to an apparatus for changing the resistance to the flow of the mud stream to create pressure pulses therein.
The usefulness of obtaining data from the bottom of an oil, gas, or geothermal well during drilling operations, without interrupting those operations, has been recognized for many years. However, no proven technology reliably provides this capability. Such a system would have numerous benefits in providing for safer and less costly drilling of both exploration and production wells.
Any system that provides measurements while drilling (MWD) must have three basic capabilities: (1) to measure the downhole parameters of interest; (2) to telemeter the resulting data to a surface receiver; and (3) to receive and interpret the telemetered data.
Of these three essential capabilities, the ability to telemeter data to the surface is currently the limiting factor in the development of an MWD system. The use of bottom-hole recorders has demonstrated the ability of currently available sensors to continuously measure the bottom-hole environment.
For safety, it is of interest to predict the approach of high-pressure zones to allow the execution of the proper kick preventative procedures. A downhole temperature sensor and gamma-ray log would be useful for this prediction. The downhole sensing of a kick would give the driller an earlier, more accurate warning than is currently available in this potentially dangerous situation. To save time and significantly reduce costs, continuous measurement of the drill bit's position would be useful during directional drilling operations.
While several downhole sensors are in general field use, none provide a signal to the surface without interrupting the drilling operation or requiring special "trips" be made when the drill string length is to be changed.
Four general methods are being studied that would provide transmission of precise data from one end of the well bore to the other: mud pressure pulse, hard wire, electromagnetic waves, and acoustic methods. At this time, the mud-pressure-pulse method seems to be closest to becoming commercially available.
The method currently being pursued to generate mud pressure pulses involves the use of a mechanical valve to modulate the resistance to the flow of the mud through the drill string. The advantages of this method are a relatively high-speed signal transmission (about 4000 to 5000 feet per second) and ready adaptability to existing equipment. (The only required modification to downhole equipment is the addition of a special drill collar near the bit that contains the pressure-pulse generating valve, the downhole sensors, and the related control apparatus). The disadvantages of this method are a relatively slow data rate (from 6 to 60 seconds for each measurement) and the poor reliability of mechanically moving parts exposed to the downhole environment.